Remove the deprecated `core.transforms.apply_transform()` function

[riley-harper]

While working on #205, we've moved hlink.linking.core.transforms.apply_transform() over to the hlink.linking.core.column_mapping module and deprecated the old apply_transform() function. Eventually we should remove the deprecated version, just keeping the version in column_mapping. There are some tests in hlink.tests.core.transforms_test which call the deprecated version and will break when it's removed. These tests are duplicated in the column_mapping_test file, so they should be OK to remove. It'd be good to double-check that we aren't losing test coverage when we do this, though.

This is a breaking change.